sometimes I see a variation, where if I play my knight for example, there is only one way for my opponent to avoid mate in three. In most cases, I play the knight, even though I know that there is a way to avoid it, just because there is a chance that he will overlook the mate. So yea, I sometimes make unsound moves!
knowingly playing unsound moves?
Sometimes it's possible to bluff bold, unsound moves when facing a player of lower strength. Just make sure you don't get caught :)

I played two 5/0 games against an opponent, who played f-pawn first and brought his king forward on its square. I lost the first game on time, but later managed to beat him in the second.
I checked his database and found dozens of games like that (no wonder that he defended with lightning speed and precision). He had a reasonable success with it in 5/0.
I later ran it through the computer and found that the opponent is only half a point worse. Though the move seemed completely dumb the other side cannot take a real advantage of it up until the late middlegame or even the endgame, thus it was a practical bet.

The idea is very simple: it's a 5/0 game; the opponent will immediately begin searching for a refutation, quick mate or material gain. It does not exist.
In the end you both arrive to an end game or late middle game, where he has 4 minutes left, and the attacker has 1.5 to 1 minute (that was my case, when I attacked). You blunder or run out of time, he grabs another easy win.
http://www.chess.com/livechess/game.html?id=17984289
http://www.chess.com/livechess/game.html?id=17985168
He resigned in the second one. If he had not done it, I would have probably lost on time again, as I was down to 40-45 seconds.

do you guys every make moves that you know are unsound. but you just do them because you like the result, only if your opponent just moved his knight....
only when I am ready to lose! if you know youre playing an unsound move, then there's something wrong with ya! Of course, I've done it before. and I will keep doing it again and again!

Never -- I always assume my opponent will make the best possible move (that I can see). No, when I make unsound moves it's entirely unwitting.

but what if - as sebas stated- when only the perfect responce gives you a small disadvantage and all the other gIve you a mate?

Back when I was messing with blitz chess online, I would make moves strictly based on how much I thought it would complicate things for the other guy. Probably similar to what you are saying here. If given the time, I'm sure a lot of the moves I made were "unsound" if the opponent would have had the time to prove it out by exploiting my weakness. However, if I could just confuse things enough for my opponent to get into time trouble, I just had to keep making moves that didn't lead to an immediate mate. Using this strategy, I was able to win a lot of games. But, in the end, it got boring for me so I quit playing blitz.

but what if - as sebas stated- when only the perfect responce gives you a small disadvantage and all the other gIve you a mate?
I keep looking. I'll make the move if it's got the best worst case scenario out of all of the candidates, but if I find another move that doesn't have the potential for mate (or siginificant material/positional gain) and has signifigantly less potential downside I'll make that one instead. Basically, I always give my opponents the benefit of the doubt and never make an unsound move who's success depends on a blunder from my opponent.

It depends on what kind of position it is...but I play two completely unsound openings with black, if you check my record I don't think I have never played anything else, I play 1...e5 as a responce to 1.d4 and 2... Nf6 scandinavian to 1.e4. Both of those openings are unsound (the e5 one even more so) but I have a more than reasonable score with those openings, even in correspondence because I know the lines by heart and know the best posible moves that can be made in the position by my oponent, I have learned how to play the endgames that can dirive from those positions, know the pawn structures inside-out, and most importantly I know how to severely punish and capitalize when my oponent makes anything other than the best of the best moves, so I guess it can work out to play unsoundly, It's a matter of preparation.

styxtwo> do you guys every make moves that you know are unsound. but you just do them because you like the result, only if your opponent just moved his knight....
No, I rarely play moves that I know my opponent can refute unless I'm already losing. However, I will play risky moves with no clear refutation and I will play 2nd or 3rd best moves that give my opponent more chances to go wrong.

For example, in a blitz game in this position:
I had to choose between 9...g4 to prod his knight, 9...dxc3 to set a trap, and 9...Be6 to develop and castle. I chose the second option because my opponent was 1100. In the best case I win immediately and in the worst case I still win.

i always play unsound moves if the position will look beautiful. I follow Gufeld's idea of the Mona Lisa in chess.

Only play unsound moves (or traps) when you're losing. Since you have nothing to lose anyway, if you think it's a hard to see the trap, go for it. These things tend to catch people who are winning because they think they control everything. If they find it, you will lose more quickly, but you were probably going to lose anyway. If you're not losing, I guess you could try but I don't see the point of giving your opponent the chance capitalize on an unsound move when you could have a perfectly fine position and just outplay him. I figure that if I saw it was unsound, they will probably see it too. Gambits that are ultimately unsound aren't necessarily bad since it can be tough on an opponent (especially one who doesn't have a tactical style, like me). Gambits tend not to lose by force anyway, so for practice and original play they are good choices. They can also be successful in tournament play, but you have to hope they can't refute it rather then outplay them with no risk. And if someone used it at the world championship for example, I'm sure the opponent could defuse the attack and make good use of that extra pawn!
do you guys every make moves that you know are unsound. but you just do them because you like the result, only if your opponent just moved his knight....